Breast cancer is expected to cause 44,500 deaths in 1991 and will occur in 1 of 9 women over their lifetime. Epidemiologic studies consistently demonstrate that one of the most important predictors of a women's risk of breast cancer is the presence of the disease in her immediate family. A total of 100 female first degree relatives of women with breast cancer participating in the Iowa Women's Health Study were asked to give a blood sample for genetic linkage analysis.